In recent years, GaN-based compound semiconductors have attracted attention as a semiconductor material for short-wavelength light-emitting elements. A GaN-based compound semiconductor is generally formed on a substrate of single crystal sapphire, various oxides, group III-V compounds, or the like by thin film forming means such as an organic metal chemical vapor deposition method (an MOCVD method) and a molecular-beam epitaxy method (an MBE method).
A GaN-based compound semiconductor generally comprises: a laminated semiconductor layer composed of an n-type GaN-based compound semiconductor layer (hereinafter, n-type semiconductor layer), active layer (hereinafter, light-emitting layer) and p-type GaN-based compound semiconductor layer (hereinafter, p-type semiconductor layer); a p-type electrode on the p-type semiconductor layer; and an n-type electrode on the n-type semiconductor layer.
Such GaN-based compound semiconductor light-emitting elements include so-called flip-chip semiconductor light-emitting elements in which a light-emitting element is mounted on a circuit board with a substrate facing up and an electrode facing down so that light emitted from a light-emitting layer is extracted outside through the substrate.
An electrode having a reflection layer of Ag, Al or the like is formed in a flip-chip semiconductor light-emitting element, in order to reflect the light with the electrode. However, Ag and Al are metals which are likely to be oxidized, and there is a problem in that they are deteriorated by contact with external moisture and air.
Therefore, various methods have been proposed to prevent the deterioration. For example, in order to prevent contact with external moisture and air, it has been proposed to form an insulation protective layer of silicon oxide, silicon nitride or the like on the surfaces and surroundings of the p-type and n-type electrodes. Also, in order to improve insulation between the p- and n-type electrodes, it has been proposed to form an insulation protective layer between the electrodes.
Au is generally used for the outermost surface of the electrodes from the viewpoint of ease of wiring. However, since adhesion of the above silicon oxide and silicon nitride with Au is very poor, there has been a problem in that it is impossible to form a strong protective layer on the electrode. In order to solve this problem, disclosed techniques are to form an adhesion enhancing layer between Au and the insulation protective layer, so as to improve adhesion (Patent Documents 1 and 2).
However, even if adhesion between Au and the insulating protective layer is improved, there is still a problem of detachment of electrodes and low reliability of light-emitting elements in cases where adhesion is insufficient between metal of the electrode and the GaN-based compound semiconductor.
Techniques are also disclosed that surround Ag with a metal which does not react with Ag in order to use Ag, which is susceptible to oxidation and migration at a high-temperature range of 300° C. or higher (Patent Document 3), and use a platinum group metal having high reflectivity or an alloy thereof as a reflection layer (Patent Document 4).
However, even if Ag is surrounded and covered by a metal which does not react with Ag, the effect is not sufficient in cases where an Ag electrode and GaN-based compound semiconductor contact with each other in a large area. That is, when the contact area is large, there have been problems in that Ag migration to the GaN-based compound semiconductor occurs, which results in a decrease in luminous efficacy due to a decrease in reflectivity and also results in an increase in VF due to poor contact. Further, although Pt group metals are resistant to migration, they have originally lower reflectivity than Ag and it is impossible to prevent migration at a temperature reaching nearly 400° C.